It was not too long ago that email mailboxes were so full of junk mail and spam that they threatened to render electronic communication useless. When you opened up your email you were bombarded with poorly written advertisements for $ex, V!agra, and tons of other intentionally misspelled products, designed to evade any spam blocking devices. Those interested in consumer protection knew the ultimate goal, to eliminate and block spam, but as soon as they created a product designed to do just that, the spammers evaded their efforts by getting more creative. That is, until modern anti spam software was developed. Antispam software comes in a variety of forms, with the obvious ultimate intent of stopping unwanted emails from reaching you.
Blacklist
One of the primary anti spam methods is known as blacklisting.

This software identifies the IP address of the spam sender, and then communicates with the Internet Service Provider of the sender and instructs the ISP to block mail from that IP address to your email account. In theory this is a fool proof solution. The reality, however, is that there is a lot of money to be made in spamming, so forcing a spammer to switch his IP address frequently is not too high a price to pay to evade blocking. That said, this practice does, over time, start to close down doors to spammers and all but eliminates amateur spammers who do not have the capability to frequently switch IPs.
Spam Votes
Many individuals who frequently use their email accounts will be familiar with this device.

Spam voting software works through the participation of users. When you receive email you have the option of classifying it as spam, usually by pushing a button which says, unsurprisingly, ‘spam’. Once enough people classify a piece of mail or an IP as spam it falls in trust until ultimately it becomes completely blocked from addresses.
Profiling
Profiling involves learning the common characteristics of spammers and spam mail. It is software that looks for things like bugs, invalid message ID’s and other traits and uses these characteristics to evaluate incoming pieces of mail. Each piece of mail is then given a score depending upon how it fares against these criteria. The user is then given the option of how high or how low to set the bar with regard to which emails are let in. This method has been shown to be immensely effective against amateur spammers and many professional spammers. However, it relies upon a ready team of professionals to identify new traits used by spammers and to incorporate those traits into the profiling algorithms.
Bayesian Filtering
The most promising spam blocking software follows no rules.

Rather, it constantly learns new techniques to fight spam by scanning the mail you’ve read and comparing it to the mail that you have rejected. This highly sophisticated software uses the data that it gleans from thousands of users to identify which items are spam and which are not. It then has the capability to adjust its standards to your particular preferences. Over time, it becomes adept at sending you only the emails that you want, and blocking the emails that you do not.